Conventional automated material handling systems used in semiconductor fabrication generally perform two basic functions: (1) storing material between processing steps, and (2) transporting material between storage locations and/or processing locations. Historically, the storage or stocking function has been accomplished by stacking horizontal rows of storage shelves vertically, as if attached to a flat, vertical wall, to provide a “vertical array” of storage locations, then providing a multi-axis robot which can move to each shelf location and place or remove a wafer carrier at that location. To increase robot utilization, a second wall of shelves is often provided on the opposite side of the robot from the first, and the robot can then access storage locations in either direction. Several input and output ports are typically provided within these walls of shelves to allow handoffs to the material transport system. These typically each consist of a secondary robot replacing several shelves, which can receive a wafer carrier from the primary stocker robot and move it through the wall of shelves to an exterior position where it can be accessed by the transport system.
FIG. 1 illustrates one embodiment of a conventional stocker 1 for storing semiconductor wafers 11 in cassettes 14 in storage bins 16. The stocker 1 includes a plurality of vertical rows 18 of storage bins 16, each of which can be accessed by a transport mechanism 20, including a rotatable elevator 22 on which a carriage 24 is mounted for vertical movement. The carriage 24 carries horizontally movable transport arms 26 for engaging the cassettes 14 under side lips 28 of the cassettes 14. Each vertical row 18 of the storage bins 16 includes a source of pressurized, particle-free air directed through a respective vertical distribution system 30. Such a stocker is disclosed in U.S. Pat. No. 5,059,079, entitled “Particle-Free Storage for Articles,” which is assigned to Asyst Technologies, Inc., and is incorporated in its entirety herein.
This conventional stocker construction has several disadvantages. First, it requires a robot with a large moving mass and a large vertical range of motion, which in turn requires a heavy, expensive support structure and drive system. Second, this large mass limits the speed with which the robot can move, which limits how fast the robot can store or retrieve a wafer carrier. Third, the large mass of the robot base and shelf structures typically require them to be mounted on the clean room floor, increasing the total clean room floor space required for a given factory output. The cost of this additional clean room space often approaches the cost of the stocker itself.
Therefore, there is a need to store wafer containers in a horizontal array to reduce the mass of the stocking robot, reduce the required vertical travel through which the robot and carrier mass must be moved, to eliminate the need to provide additional clean room floor space for the purpose of wafer carrier storage, and to provide a larger number of input and output ports to the stocker.